1. Field of the Invention
The present invention relates to a method of ironing a cylindrical portion of a cylindrical workpiece formed of austenite stainless steel, and more particularly to such an ironing method which permits improved accuracy of outside and inside diameters of the ironed cylindrical portion.
2. Discussion of the Prior Art
A cylindrical portion of a cylindrical workpiece which has a bottom portion at one of the opposite axial ends of the cylindrical portion is ironed for improving the accuracy of the diametral dimensions (outside and inside diameters). The ironed cylindrical workpiece is used as a container, for example. This ironing process uses a punch, and a die which has a die hole formed therein so as to provide a land portion, which cooperates with the punch to effect an ironing operation. The punch is inserted into the cylindrical portion of the workpiece, through the open end of the cylindrical portion, until the end face of the punch is brought into abutting contact with the bottom portion of the workpiece. In this condition, the punch is moved with the workpiece into the die hole so that the wall thickness of the cylindrical portion is reduced, with the cylindrical wall squeezed between the moving punch and the land portion of the stationary die.
It is recognized that the accuracy of the diametral dimensions of the ironed cylindrical portion of the workpiece is improved with an increase in the wall thickness reduction ratio or percent of the cylindrical portion, when the workpiece is made of an ordinary material such as SPCC, copper and aluminum. The wall thickness reduction ratio means a ratio of the initial wall thickness of the cylindrical portion of the workpiece before ironing, to the wall thickness of the ironed cylindrical portion. The wall thickness reduction ratio or percent is determined so as to attain the desired accuracy of the diametral dimensions of the ironed workpiece.
For the workpiece made of austenite stainless steel, however, there is not a recognition in the art on the relationship between the wall thickness reduction percent and the accuracy of the diametral dimensions of the ironed workpiece. The present inventors found a problem with the prior art ironing process when applied to the cylindrical workpiece made of austenite stainless steel. This problem experienced in the prior art will be described by reference to the accompanying drawings.